App: extend Expression syntax

* Support sub-object reference syntax using the following syntax,
    Part.<<Box.>>.Placement
  or, with sub-object label referencing
    Part.<<$Cube.>>.Placement

* Extend indexing support, including range based indexing, e.g.
    A1[B2+1][C3][D4:-1]

* Add new constants, None, True, true, False, false.

src/App/Application.cpp

@@ -1750,7 +1750,6 @@ void Application::initTypes(void)
     App ::ConditionalExpression     ::init();
     App ::StringExpression          ::init();
     App ::FunctionExpression        ::init();
-    App ::BooleanExpression         ::init();
     App ::RangeExpression           ::init();
     App ::PyObjectExpression        ::init();